You're going to have to explicitly define all of those routes. Otherwise you will always end up at the "user_controller".
$route['signup'] = "signup";
$route['(:any)'] = "user/display/$1";
or something similar. They are ran in order, so what ever is defined first, is going to happen first. So if you catch (:any), you're going to send ANYTHING to that controller.
Also keep in mind that you can use regular expressions, so if you know there is always going to be a '.' in there, you could test for that.